Showing papers in "Optical Engineering in 1999"

Color-encoded digital fringe projection technique for high-speed three-dimensional surface contouring

Abstract: A color-encoded digital fringe projection technique is proposed for high-speed 3-D surface contouring applications. in this technique, a color fringe pattern whose RGB components comprise three phase-shifted fringe patterns is created by software on a computer screen and then projected to an object by a novel computer-controlled digital projection system. The image of the object is captured by a digital camera positioned at an angle different from that of the projection system. The image is then separated into its RGB components, creating three phase-shifted images of the object. These three images are used to retrieve the 3-D surface contour of the object through the use of a phase wrapping and unwrapping algorithm. Only one image of the object is required to obtain the 3-D surface contour of the object. Thus contouring speed, limited only by the frame rate of the camera, can be dramatically increased as compared to that of the traditional phase-shifting techniques. The technique is especially useful in applications where the object being contoured is going through quasi-static or dynamic changes. The principle of the technique is described and some preliminary experimental results are presented.

Three-dimensional video system based on integral photography

Abstract: A three-dimensional (3-D) video system based on integral photography is proposed. In this system, real erect images of an object are formed by a gradient-index (GRIN) lens array as elemental images and are directly captured by a television camera. A large-aperture convex lens is introduced in front of the GRIN lens array to control the depth range of the reproduced image. The video signal of a group of elemental images is transmitted to a display device that combines a liquid crystal panel and an array of convex microlenses, producing a color 3-D image in real time. As a result, full-color and autostereoscopic 3-D images with full parallax can be observed. We confirmed the possibility of a 3-D television system.

Optical image encryption based on XOR operations

Abstract: Kwangwoon UniversityDepartment of Electronic Engineering447-1, Wolge-Dong, Nowon-GuSeoul 139-701South KoreaAbstract. We propose a new optical image encryption technique basedon exclusive-OR (XOR) operations for a security system that controls theentrance of authorized persons. The basic idea is that we convert agray-level image to eight bit planes for image encryption. We use theXOR operation that is commonly used such as the well-known encryp-tion method. The input image is encrypted by performing optical XORoperations with the key bit stream that are generated by digital encryp-tion algorithms. The gray level input image is converted to eight bitplanes, which are represented on a liquid crystal device (LCD). The keydata represented on different LCDs is reproduced to eight bit planes by alenslet array. The optical XOR operations between the key data and thebit planes are performed by the polarization encoding method. The re-sults of XOR operations, which are detected by a CCD camera, areconverted to an encrypted gray-level image and the image is used as aninput to the binary phase extraction joint transform correlator (BPEJTC)for comparison with reference images. We present some simulation re-sults that verify the proposed method.

Discrete cosine transform based regularized high-resolution image reconstruction algorithm

Abstract: While high-resolution images are required for various applica- tions, aliased low-resolution images are only available due to the physi- cal limitations of sensors. We propose an algorithm to reconstruct a high- resolution image from multiple aliased low-resolution images, which is based on the generalized deconvolution technique. The conventional approaches are based on the discrete Fourier transform (DFT) since the aliasing effect is easily analyzed in the frequency domain. However, the useful solution may not be available in many cases, i.e., the underdeter- mined cases or the insufficient subpixel information cases. To compen- sate for such ill-posedness, the generalized regularization is adopted in the spatial domain. Furthermore, the usage of the discrete cosine trans- form (DCT) instead of the DFT leads to a computationally efficient recon- struction algorithm. The validity of the proposed algorithm is both theo- retically and experimentally demonstrated. It is also shown that the artifact caused by inaccurate motion information is reduced by regular- ization. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00508-5)

Image quality and λFN/p for remote sensing systems

Abstract: The ratio of the sampling frequency to the optical bandpass limit of an incoherent diffraction-limited optical system is a fundamental design parameter for digital imaging systems. This ratio is denoted by ?FN/p, where ? is the mean wavelength, FN is the system f/number, and p is the detector sampling pitch. The value of ?FN/p for a remote sensing system can have a profound impact on the image quality and the utility of the acquired images. The interaction between ?FN/p and image quality is sensitive to the system design parameters such as modulation transfer function (MTF), signal-to-noise ratio (SNR), and ground sampled distance (GSD). Image simulations and analysis are presented that illustrate the changes in image quality as a function of ?FN/p. System design trades that may influence the determination of the optimal lFN/p for a remote sensing system are also discussed.

Wide-dynamic-range sensors

Abstract: Orly Yadid-Pecht, MEMBER SPIE Ben-Gurion University of the Negev Electrical and Computer Engineering Department P.O. Box 653 Beer-Sheva 84105 Israel E-mail: oyp@ee.bgu.ac.il Abstract. The work done to provide image sensors (CCDs and CMOS) with a wide dynamic range is reviewed. The different classes of solutions, which consist of logarithmic sensors, ‘‘clipped’’ sensors, multimode sensors, frequency-based sensors, and sensors with control over integration time are described. The pros and cons of each solution are discussed, and some new experimental results are shown. Active pixel sensors with a wide dynamic range are analyzed and possible future directions are pointed out. © 1999 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(99)01610-4]

Optical projection lithography at half the Rayleigh resolution limit by two-photon exposure

Abstract: In recent years, with the advent of femtosecond pulse technology, two-photon absorption has commenced to be used for exposing photo-resists. It is natural to ask then, what is the spatial resolution of two-photon lithography? There has already been some discussion of resolution limits in two-photon, scanning confocal fluorescence microscopy. We will find that ordinary two-photon exposure of photo-resist merely enhances the photographic contrast, or gamma. While this improves the spatial resolution somewhat, it does so at the expense of a requirement for tighter control over the incident light intensity. Instead, we introduce a new type of exposure system employing a multiplicity of 2-photon excitation frequencies which interfere with one another to produce a super-resolution stationary image, exhibiting a true doubling of the spatial resolution.

Study on polarizing visible light by subwavelength-period metal-stripe gratings

Abstract: One method for influencing the polarization of light is the use of wire-grid polarizers. For the visible region, this type of element can be realized as a metal-stripe grating with periods less than the wavelength. We fabricate metal-stripe gratings with periods down to 190 nm in thin chromium layers of 35-nm thickness using electron-beam lithography and ion-beam etching. A detailed investigation of the influence of grating period and duty cycle on the polarization effect is carried out to verify the conformity of rigorous diffraction theory and experimental results. The comparison between the two indicates good performance. Polarization ratios of the order of 5 with transmission efficiencies of about 60% in TM polarization are obtained. The connection of the polarization effect real- izable and the fabrication technology used is discussed. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00902-2)

Transmission variations in liquid crystal spatial light modulators caused by interference and diffraction effects

Abstract: We report on the characteristics of a newly developed high- resolution (6403480 pixels) parallel-aligned liquid crystal spatial light modulator (LCSLM) as a function of wavelength. Phase-only operation over a range of 2p rad is easily achieved by operating at shorter wave- lengths. We also measure an unexpected effect—the transmitted light intensity changes with applied voltage. Our experiments show that thin- film interference and pixel diffraction effects are responsible for this ob- served behavior. The diffraction effect is caused by a nonuniform electric field across each pixel. This nonuniform electric field introduces a blazing effect that changes the intensity distribution in the various diffracted or- ders as a function of applied voltage. These same kinds of effects have been observed with several other twisted-nematic LCSLMs. Because of the complicated polarization effects caused by these twisted-nematic liq- uid crystal devices, however, the diffraction and interference effects are more easily studied using the parallel-aligned LCSLM. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)01006-5)

High-spectral-resolution lidar using an iodine absorption filter for atmospheric measurements

Abstract: We develop high-spectral-resolution lidar (HSRL) using an io- dine absorption filter and a narrow-band Nd:YAG laser with high- frequency-doubled output and use it for atmospheric measurements at the National Institute for Environmental Studies (NIES). The lidar system separates Doppler-broadened molecular scattering and unbroadened aerosol scattering components of the backscattered laser light, conse- quently providing direct measurements of these two signals. Due to stable and strong absorption at room temperature, iodine absorption fil- ters are easy to use and they enable high rejection against aerosol scat- tering with a short length. The measurement principle with an error analysis and the system construction of the HSRL are described. Ex- amples of aerosol and cloud observations made by the HSRL are also presented. The experimental results indicate that the HSRL is a powerful tool for quantitatively measuring aerosol and cloud optical properties. The use of a high-output laser gives our system the ability to provide stratospheric observations. We also present the HSRL measurements of the stratospheric temperature and backscatter profiles with better reso- lutions in both range and time. An example of temperature and backscat- ter measurements is shown for the altitude range from ;10 to ;40 km with a vertical resolution of 300 m and an integrated time of ; 2h . © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00910-1)

Comprehensive analysis of edge detection in color image processing

Abstract: Various approaches to edge detection for color images, in- cluding techniques extended from monochrome edge detection as well as vector space approaches, are examined. In particular, edge detection techniques based on vector order statistic operators and difference vec- tor operators are studied in detail. Numerous edge detectors are ob- tained as special cases of these two classes of operators. The effect of distance measures on the performance of different color edge detectors is studied by employing distance measures other than the Euclidean norm. Variations are introduced to both the vector order statistic opera- tors and the difference vector operators to improve noise performance. They both demonstrate the ability to attenuate noise with added algo- rithm complexity. Among them, the difference vector operator with adap- tive filtering shows the most promising results. Other vector directional filtering techniques are also introduced and utilized for color edge detec- tion. Both quantitative and subjective tests are performed in evaluating the performance of the edge detectors, and a detailed comparison is presented. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00904-6)

Calibration of a parametric model of an optical microscope

Abstract: In micro-domain applications, such as assembly of hybrid mi- croelectromechanical systems, optical microscopes form a critical com- ponent of visually based microassembly systems. In order to effectively use vision feedback to guide manipulation strategies using a broad range of optical microscope configurations, accurately calibrated para- metric models of optical microscopes are required. Although many cali- bration techniques exist for macroscopic camera-lens systems, there is a dearth of literature on calibration of parametric microscope models. Op- tical microscope calibration has unique characteristics that are quite dif- ferent from normal camera calibration, including (1) unique calibration parameters; (2) calibration patterns that must be parallel to the image plane, and (3) restriction to single-plane calibration. In this paper, we propose a parametric microscope model and calibration algorithm spe- cifically for optical microscopes. Our approach extends existing camera calibration techniques to include the unique parameters of optical micro- scopes and also allows the use of a single calibration plane perpendicu- lar to the optical axis. The validity and accuracy of our proposed micro- scope calibration method are tested by experiments. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)02412-5)

Optical roughness measurements using extended white-light interferometry

Abstract: Roughness measurements of engineering surfaces are an important and interesting application for optical sensors The increased speed, the ability to measure 3-D profiles, and the noncontact mode of optical sensors are of great interest for practical uses A most challeng- ing task is achieving comparability to the established methods, such as tactile stylus sensors, which are the most commonly used instruments in industry We show that white-light interferometry is a useful method to measure in a microscopic range as well as with an extended field of view The determination of the fringe contrast as a function of the focus position enables highly resolved data acquisition on engineering sur- faces with a mean roughness of the order of a few nanometers to several micrometers The measurements show that amplitude roughness pa- rameters of engineering surfaces are not strongly affected by the spatial resolution A good agreement between the results of a stylus sensor and the results of our optical method are demonstrated on different types of surfaces © 1999 Society of Photo-Optical Instrumentation Engineers (S0091-3286(99)00906-X)

Fourier transform method for automatic processing of moire deflectograms

Abstract: In this work, a Fourier transform technique for automatic analysis of moire deflectograms is presented. A squared grating is used to multiplex the information of the deflection in two orthogonal directions in one image. This procedure avoids the necessity of rotating the gratings to obtain the complete deflection information. With this method only two fringe patterns. reference and distorted, are needed to determine the complete deflection information. To deal with irregularly shaped processing areas, a Gerchberg extrapolation method is used. The automatic determination of the carrier as well as the size and position of the reconstruction windows permit the complete and automatic measurement of the deflection produced by an ophthalmic lens in two orthogonal directions. Afterwards, the refractive power maps can be obtained, Experimental results obtained with a progressive addition lens are presented and comparison with measurements obtained with a commercial focimeter are shown showing a good agreement.

Three-dimensional multicriterion automatic segmentation of pulmonary nodules of helical computed tomography images

Abstract: A 3-D multicriterion automatic segmentation algorithm is de- veloped to improve accuracy of delineation of pulmonary nodules on helical computed tomography (CT) images by removing their adjacent structures. The algorithm applies multiple gray-value thresholds to a nodule region of interest (ROI). At each threshold level, the nodule can- didate in the ROI is automatically detected by labeling 3-D connected components, followed by a 3-D morphologic opening operation. Once the nodule candidate is found, its two specific parameters, gradient strength of the nodule surface and a 3-D shape compactness factor, can be computed. The optimal threshold can be determined by analyzing these parameters. Our experiments with in vivo nodules demonstrate the feasibility of employing this algorithm to improve the accuracy of nodule delineation, especially for small nodules less than 1 cm in diameter. This discloses the potential of the algorithm for accurate characterizations of nodules (e.g., volume, change in volume over time) at an early stage, which can help to provide valuable guidance for further clinical manage- ment. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)00708-4)

Minimum temperature difference perceived--a new approach to assess undersampled thermal imagers

Abstract: The minimum temperature difference perceived (MTDP) is a new device figure of merit for assessing undersampled thermal imagers MTDP is based on the perception of the standard four-bar test pattern, like minimum resolvable temperature difference (MRTD) In undersampled imagers, the test pattern image is increasingly distorted toward higher pattern frequencies due to aliasing Contrary to MRTD, MTDP takes into account such signal distortions to a certain extent With MTDP the imager performance can be assessed beyond the Nyquist frequency The modulation in the distorted four-bar pattern images is studied as a function of pattern frequency and position We show that for each pattern frequency, an optimum phase position can be found at which the average modulation in the reproduced pattern image has a maximum This modulation is defined as the average modulation at optimum phase (AMOP) The MTDP is the temperature difference at which four, three or two bars can be resolved by an observer, with the test pattern at the optimum phase position The MTDP approach is implemented in the TRM3 model Examples comparing measured and predicted MTDP data for several staring imagers are presented The validity of MTDP to predict the range performance of undersampled imagers is examined and results are presented

Stress, reflectance, and temporal stability of sputter-deposited Mo/Si and Mo/Be multilayer films for extreme ultraviolet lithography

Abstract: Precisely figured optics coated with high-reflectance multilayer films are an integral part of an extreme ultraviolet (EUV) projection lithography system. Since multilayer film stress deforms these precisely figured optics, it is important that this stress be characterized and that suitable methods are developed to negate the effects of the stress. However, these techniques must reduce or compensate for the effects of film stress without significantly reducing the EUV reflectivity, since the reflectivity has a strong impact on the throughput of an EUV lithography system. Different techniques for reducing multilayer stress are examined. The technique of varying the base pressure (impurity level) yielded a 10{percent} decrease in stress with a 2{percent} decrease in reflectance for our multilayers. A study of annealing during Mo/Si deposition is performed; a stress reduction of 70{percent} is observed at 200{degree}C, similar to what has been found for postdeposition annealing. However, the reflectance loss was 3.9{percent} versus 1.3{percent} for postdeposition annealing, indicating that if annealing is performed on our films it should be done after multilayer deposition. A decrease of approximately 9{percent} in the bilayer period thickness was observed for annealing during deposition at temperatures near 120 to 140{degree}C, much larger than the thickness changes observed duringmore » postdeposition annealing. A new athermal buffer-layer technique is developed to compensate for the effects of stress. Using this technique with amorphous silicon and Mo/Be buffer layers it is possible to obtain Mo/Be and Mo/Si multilayer films with a near zero net film stress and less than a 1{percent} degradation in reflectivity. It is important that the multilayer coated optics are stable with time and that the stress reduction and compensation techniques do not degrade the stability. The temporal stability of 100-nm-thick Mo, Si, and Be single-layer films, Mo/Si and Mo/Be multilayer films, and films treated with various stress reduction/compensation techniques are investigated. Results are reported showing the reflectance peak wavelength to be stable to within 0.15{percent} for both the multilayers and the stress-compensated multilayers. Therefore, stability is maintained when these buffer-layer stress compensation techniques are applied. Mo/Si multilayer films exhibit a {approximately}10{percent} short-term decrease in the stress over the first few months after deposition, but the stress stabilizes thereafter. The Mo/Si reflectance is observed to be stable to within 0.4{percent} over a period of over 400 days. The Mo/Be multilayer film stress was stable to within about 2{percent} over 300 days, and the reflectance was observed to decrease by about 1.5{percent} over the same period. For Mo/Si on a Mo/Be buffer layer the stress changed from {minus}28 to +3 MPa, and the reflectance decreased by approximately 0.4{percent} for a period of over 300 days. For Mo/Be on an a-Si buffer layer, the stress changed from {minus}23 to {minus}3 MPa, and the reflectance decreased by {approximately}1.8{percent} for a period of over 300 days; this drop in reflectance is believed to be due primarily to the Mo/Be and not the addition of the buffer layer. {copyright} {ital 1999 Society of Photo-Optical Instrumentation Engineers.}« less

Influence of sampling on target recognition and identification

Abstract: Two perception experiments are conducted to quantify the relationship between imager sampling artifacts and target recognition and identification performance using that imager. The results of these experiments show that in-band aliasing (aliasing that overlaps the baseband signal) does not degrade target identification performance, but out- of-band aliasing (such as visible display raster) degrades identification performance significantly. Aliasing had less impact on the recognition task than the identification task, but both in-band and out-of-band aliasing moderately degrades recognition performance. Based on these experiments and other results reported in the literature, it appears that in-band aliasing has a strong effect on low-level discrimination tasks such as point (hot-spot) detection; out-of-band aliasing has only a minor impact on these tasks. For high-level discrimination tasks such as target identification, however, out-of-band aliasing has a significant impact on performance, whereas in-band aliasing has a minor affect. For intermediate-level discrimination tasks such as target recognition, both in-band and out-of-band aliasing have a moderate impact on performance. Based on data from the perception experiments, the modulation transferfunction (MTF) squeeze model is developed. The degraded performance due to undersampling is modeled as an effective increase in system blur or, equivalently, a contraction or "squeeze" in the MTF. An equation is developed that quantifies the amount of MTF squeeze or contraction to apply to the system MTF to account for the performance degradation caused by sampling.

Modeling and measurements of atmospheric optical turbulence over land

Abstract: The effects of atmospheric turbulence on optical system performance depend primarily on the refractive index structure parameter C2n and the inner scale ?0. A simple model for C2n and ?0 is described and experimentally tested. The model, based on the Monin-Obuhkov similarity theory, was developed by Thiermann and is valid in the atmospheric surface layer over land. The inputs to the model are standard meteorological parameters plus solar irradiance and parameters that characterize the local terrain. Model results are compared with measurements of C2n and ?0 made with a laser scintillometer at a height of 1.8 m. The performance of the model was found to be good during unstable daytime conditions.

Ambiguities in measuring the physical parameters for twisted-nematic liquid crystal spatial light modulators

Abstract: There is great interest in using commercially available twisted-nematic liquid crystal displays as programmable spatial light modulators. Proper use of such displays, particularly as phase-only filters for optical correlators or as diffractive optical elements, requires knowledge of the physical parameters of the device including the twist angle, the birefringence, and the orientation of the director axes. A simple experimental technique was published for evaluation of these parameters. When we used this technique, however, we found a number of different sets of values that satisfied the experimental data. We demonstrate that additional measurements at different wavelengths can resolve these ambiguities and provide valuable information about the wavelength dependence ofthe birefringence.

Understanding image quality losses due to smear in high-resolution remote sensing imaging systems

Abstract: Space-based high-resolution scanning array imaging sys- tems have the potential to introduce large amounts of image smear. When designing these systems, it is useful to understand how smear will degrade image quality. A brief description of the causes of smear and a simple mathematical model are presented. A series of image simulations (for a system in which lFN/p51.0, where l is the mean wavelength for a panchromatic system, FN is the system f number, and p is the pixel pitch of the detectors) are performed in which along scan smear (ranging from 1.0 to 8.0 pixels) is introduced. Using the National Imagery Inter- pretability Rating Scale (NIIRS), expert observers rated DNIIRS differ- ence in image quality between the images with simulated smear and the original ''unsmeared'' image. The functional relationship between smear error and image quality (in units of DNIIRS) is determined. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)01505-6)

Three-dimensional imaging system

Abstract: A three-dimensional imaging system is described which exploits the defocusing of non-zero diffraction order images caused by the quadratic distortion of a diffraction grating (4) An optical system (1) is used such that objects (5, 6 and 7), located at different distances from grating (4), are imaged simultaneously and spatially separated on a single plane B

Novel implementation of nonlinear joint transform correlators in optical security and validation

Abstract: Emblems using holograms or other diffractive devices have long been used to mark cards and other objects as a means of authentication. The effectiveness of such emblems as a security device is ultimately determined by the inspection system. Due to the expense and highly variable performance of the human inspector, automated machine reading devices are an attractive alternative for performing the verification task. An additional advantage of the machine reader is that information regarding the card or its holder may be stored covertly. A security verification system is presented consisting of a holographic security emblem in which information is covertly stored, and an automated reader based on a joint transform correlator (JTC). A holographic encoding method is used to produce an emblem that stores the required phase and/or amplitude information in the form of a complex, 3-D diffraction pattern that can only be interpreted through the use of a second 'key' hologram. The reader incorporates the use of the non-linear material, Bacteriorhodopsin, as a means of miniaturizing the system, reducing system cost, and improving system performance. Experimental results are presented that demonstrate the feasibility of the approach for security applications.

Multiprimary color display for liquid crystal display projectors using diffraction grating

Abstract: To reproduce the natural color of an object through color im- aging systems, the range of the reproducible color, i.e., the color gamut, of the color display devices must be expanded, because the color gamut of current display devices such as cathode ray tubes (CRTs) and liquid crystal displays (LCDs) is insufficient. We present a new method to en- large the color gamut for the LCD device using more than three prima- ries, i.e., multiprimaries, generated by a diffraction grating and a liquid crystal panel. The optical system for the multiprimary color display is introduced, and its capability of increasing the gamut is discussed. The experimental result displayed by seven primary colors is also demon- strated. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)02011-5)

Fabric wrinkle evaluation using laser triangulation and neural network classifier

Abstract: Wrinkling is one of the most important fabric performance properties, which are routinely evaluated in reference to a set of visual standards in the textile industry. The visual evaluation is unreliable and time-inefficient. An industrial need for objective and automatic evaluation methods has been increasing markedly in the recent years. An image analysis system is developed to meet this need. The laser line triangu- lation method is used to measure the 3D surface data of a wrinkled fabric, and a neural network is built to execute the wrinkle classification with respect to the visual standard. Due to the directionality of wrinkles, a rotary stage is used to change the sample's orientation under the cam- era so that multiple images of the sample can be captured at different angles and more wrinkle data can be extracted for classification. The wrinkle classifications provided by the system are highly consistent with the visual standards, showing the potential for replacing human graders in fabric wrinkling evaluations. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)01810-3)